Add pagination blocks, renderCore, layout audit and react-pdf quirk tests

TableGuard, BottomSpacer, FirstPageHeader and PageChrome implement the
verified pagination rules; renderCore returns bytes, a normalised layout
tree, audit issues and a layout fingerprint. auditLayout detects overlaps,
ink overflow, truncation, header-only pages and split blocks. Quirk tests
pin the react-pdf behaviours the design depends on, and a child-process
test pins the page-level absolute Svg hang.

src/pdf/engine/metrics.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from "vitest";
+import { chromeHeight } from "./metrics";
+
+describe("chromeHeight", () => {
+  it("is ceil((ascent - descent + lineGap) / upm * size) + 1 from the manifest", () => {
+    // Plex: 1025 + 275 = 1300 / 1000 = 1.3 em
+    expect(chromeHeight({ face: "IBM Plex Sans", weight: 400, size: 8 })).toBe(Math.ceil(10.4) + 1);
+    expect(chromeHeight("footer")).toBe(Math.ceil(9.75) + 1);
+    expect(chromeHeight({ face: "IBM Plex Sans", weight: 400, size: 10 })).toBe(14);
+  });
+
+  it("covers the line react-pdf draws for render-prop text", () => {
+    // A 8 pt Plex line is 10.4 pt tall (verified in the layout tree); the reserve must be at least that.
+    expect(chromeHeight({ face: "IBM Plex Sans", weight: 400, size: 8 })).toBeGreaterThanOrEqual(10.4);
+  });
+
+  it("falls back across weights and styles instead of failing", () => {
+    expect(chromeHeight({ face: "IBM Plex Sans", weight: 600, italic: true, size: 8 })).toBe(12);
+  });
+});